Abstract
•We studied Abderaz Formation in four sections of the Kopeh-Dagh basin north Iran.•Nine planktonic biozones were recorded.•Some macrofauna was recorded in the studied interval.•Three stage boundaries: T/C, C/S and S/C were detected.
The foraminiferal content of the well-known, foraminifera-rich Abderaz Formation of the Kopeh-Dagh basin in northeastern Iran was analyzed to determine biostratigraphy, age dating, chronostratigraphy and evolutionary trends. The Abderaz Formation was sampled at the following four sections: the Qarehso section, the Abderaz village section, the Muzduran section and the Shorab section. The Qarehso section is approximately 540 m thick and consists of 77 planktonic foraminifera species belonging to 18 genera in 5 biozones, 10 echinoid species belonging to 4 genera and 8 inoceramid species belonging to 5 genera. Based on this fossil content, the Qarehso section was dated to the lowermost Turonian to the upper Santonian. Based on the first occurrence datums of Cremnoceramus walterdorfensis walterdorfensis and Cremnoceramus deformis deformis and on the FODs of Micraster cortestudinarium and Dicarinella concavata, the Turonian–Conician boundary was detected in this section. In addition, based on the FODs of Echinocorys gr. scutata forma vulgaris, Echinocorys gr. scutata forma vulgaris-planodama and Di. Asymetrica, the base of the Santonian stage was also detected in the Qarehso section. The Abderaz village section is 556.5 m thick and contains 17 genera and 68 species of planktonic foraminifera that have been identified in 9 biozones and 7 genera and 15 species of invertebrates. The lowermost Turonian to the lowermost Campanian ages have been established in the Abderaz Formation. In the Abderaz village section, 3 boundaries were identified. Based on the FODs of Cremnoceramus walterdorfensis walterdorfensis, C. deformis deformis and C. crassus crassus and on the FODs of Contusotruncana fornicata and C. pateliformis, the base of the Coniacian stage was identified. Based on the FODs of Di. asymetrica and Macroglobigerinelloides alvarezi, the Coniacian–Santonian boundary was identified. Additionally, based on the extinction of the Marginotruncanid group and on the LOD of Pseudotextularia nuttalii, the base of the Campanian stage was identified. The Muzduran section of the Abderaz Formation is 400 m thick and contains 59 planktonic foraminifera species belonging to 19 genera, 10 inoceramid species belonging to 5 genera, 5 echinoid species belonging to 3 genera and 2 ammonite species belonging to 2 genera. Also based on this fossil content, the age of the formation is estimated to be the middle Turonian–early Campanian. In the Muzduran section, the base of the Coniacian stage was detected just below the first occurrence of C. deformis deformis, whereas the bases of the Santonian and Campanian stages were found at the first occurrence of Echinocorys ex gr. scutata and the LOD of the Marginotruncanids groups and of Ventilabrella austiniana, respectively. The Shorab section is 360 m thick. In this section, 51 planktonic foraminifera species belonging to 16 genera were identified, and 5 biozones were differentiated. In addition, 1 ammonite species and 8 inoceramid species belonging to 4 genera were found in this section. Based on this fossil assemblage, the age of the Shorab section dates to the lowermost Turonian–early Campanian. In this section, the study of macro- and microfauna shows three boundaries: the Turonian–Coniacian, Coniacian–Santonian and Santonian–Campanian boundaries. The base of the Coniacian stage was identified based on the FODs of the planktonic foraminifera Archaeoglobigerina cretacea and Dicarinella concavata, along with the FOD of Cremnoceramus walterdorfensis walterdorfensis of the inoceramid group, whereas the base of the Santonian stage was determined using the LOD of Dicarinella primitiva of the Whiteinellids group and the FODs of Dicarinella asymetrica and Inoceramus aff. vistulensis (an inoceramid species). The FODs of Globotruncanita elevata and Ventilabrella austiniana, along with the extinction of the Marginotruncanid group, were used to identify the base of the Campanian stage in the Shorab section.